1. Field of the Invention
This invention relates to an image forming apparatus using an electrophotographic printing method or an electrostatic recording method such as a copying machine, a printer or a facsimile apparatus.
2. Related Background Art
In conventional image forming apparatuses using an electrophotographic printing method, and above all, an image forming apparatus which effects color image formation, there is widely utilized a two-component developing method using a mixture of a nonmagnetic toner (toner) and a magnetic carrier (carrier) as a developer. The two-component developing method, as compared with other developing methods proposed at present, has such merits as the stability of the quality of image and the durability of the apparatus.
As an example of a conventional popular developing apparatus adopting the two-component developing method, mention may be made, for example, of one described in Japanese Patent Application Laid-open No. H11-007189. A two-component developing apparatus will now be described with reference to FIG. 6 of the accompanying drawings.
In such a two-component developing apparatus 1 as described in Japanese Patent Application Laid-open No. H11-007189, a developing sleeve 9 which is a developer carrying member has the function of holding a two-component developer 3 on the surface thereof by a magnetic force, and carrying it to a developing region 25 opposed to a drum-shaped electrophotographic photosensitive member (hereinafter referred to as the “photosensitive drum”) 100 as an image bearing member, and is disposed with a predetermined clearance with respect to the surface of the photosensitive drum, and has the function of applying a predetermined developing electric field to the clearance to thereby cause the toner to adhere to an electrostatic image.
As the radius of the developing sleeve 9, the order of 8 to 12 mm has heretofore been the mainstream, but in recent years, developing sleeves having a radius of about 6 mm have also been produced.
Also, as the material of the developing sleeve 9, as described in Japanese Patent Application Laid-open No. H11-007189, use is made of a nonmagnetic material such as aluminum or nonmagnetic stainless steel, but in recent years, aluminum has become the mainstream with the tendency toward a lower price.
On the other hand, due to a rise in the needs for color images in recent years, in image forming apparatuses for effecting color image formation as well as monochromatic image forming apparatuses, the downsizing of the apparatus has come to be demanded with a lower cost of the apparatus. Particularly, as regards a developing apparatus, developing apparatuses for four colors are usually provided in an image forming apparatus which effects color image formation and therefore, the necessity of the downsizing thereof is high. Of course, the developing sleeve has been in the tendency toward downsizing and a smaller radius.
The developing sleeve 9, however, tends to become lower in its mechanical strength with the tendency toward a smaller radius. The reason for this is, first, that the radius simply becomes smaller. The next reason is that in order to keep the magnetic force of a magnet roller 7 contained in the developing sleeve at a desired level, the radius of the magnet roller 7 also cannot help assuming a size larger than a certain degree of size and therefore, the inner diameter of the developing sleeve 9 cannot be made small and accordingly, the thickness of the developing sleeve must avoidably be made small.
Now, the inventor has found the presence of the following new problem in the process of his development of a compact developing apparatus.
It is the phenomenon that the developing sleeve is made to carry thereon a two-component developer having magnetism, whereby the developing sleeve is elastically deformed.
The inventor has carried out the following measurement by the use of a developing apparatus of such a construction as described in Japanese Patent Application Laid-open No. H11-007189.
First, the shape of a developing sleeve in the developing apparatus has been measured in a state before a two-component developer supplied into the developing apparatus. The measuring apparatus used is a CNC three-dimensional measuring machine Crysta-Apex 1220 produced by Mitsutoyo Corporation.
In FIG. 6, the photosensitive drum 100 was detached, and three or more positions on the circumference of the developing sleeve 9 on the side thereof opposed to the photosensitive drum were measured. From the result of this measurement, the central position of a circle in the cross section of the developing sleeve 9 shown in FIG. 6 is calculated. This measurement is carried out over several locations in a direction perpendicular to the plane of the drawing sheet of FIG. 6 (the rotation axial direction of the developing sleeve 9 in such a manner as to include the end portions and center of the image forming width region of the image forming apparatus.
Next, the shape of the developing sleeve 9 in a state in which the developing sleeve 9 is coated with the two-component developer is measured.
First, the two-component developer is supplied into the developing apparatus, and the developing apparatus is driven as it is usually used, thereby bringing about a steady state in which the developing sleeve 9 has been coated with a predetermined amount of two-component developer. The driving of the developing apparatus is once stopped for the purpose of measurement, and the two-component developer on a portion of the developing sleeve 9 on the side thereof opposed to the photosensitive drum 100 of which the position is desired to be measured is partly removed. The size of the tip end of a probe used for the measurement is a size of the order of several millimeters and therefore, the range to be removed can be an area of the order of 10 mm square so that this probe can contact with the developing sleeve 9 without contacting with the two-component developer. The method of removing may be by quietly moving the two-component developer from the measuring portion by the use of a piece of paper, an applicator or the like, or may be suction or a blast by air, or attraction by a magnet. The toner directly adhering to the developing sleeve 9 can be removed by an air blast.
By such a method, the shape of the developing sleeve 9 with the two-component developer adhering thereto can be measured. However, if the two-component developer is removed too much, there is the possibility of causing the occurrence of a condition differing from a state originally desired to be measured and therefore, care must be taken.
When the inventor measured a change in the shape of the developing sleeve 9 due to the presence or absence of the two-component developer adhering thereto, in the manner as described above, it has been found that the central axis of a cylinder formed by the developing sleeve 9 is flexed in the form of a bow.
When the inventor then measured this difference under several conditions, the amount of deformation was generally a magnitude of the order of 0.010 mm to 0.200 mm in terms of the difference between the amounts of displacement of the circles at the both ends and center of the image forming width (hereinafter referred to as the “flexure amount δ”).
The force producing this deformation is a force with which a magnet roller 7 magnetically attracts the two-component developer which is a magnetic material.
That is, the two-component developer having received the force with which it is attracted by the magnet roller 7 pushes the blank tube of the developing sleeve 9 and the circumferential distribution thereof is not uniform but is biased and therefore, by the resultant force thereof, the developing sleeve 9 is displaced in one direction.
Also, this force is axially uniformly applied in the region coated with the two-component developer and therefore, the developing sleeve 9 supported at its both ends is flexed in the form of a bow.
The flexure amount δ is changed in its magnitude and the direction of flexure by various conditions such as the material, shape and thickness of the blank tube of the sleeve, as well as the polar disposition and size of the magnet, the amount of magnetization of the magnetic carrier, and the amount of adhesion of the two-component developer attributable to the position of a regulating blade and the shape of a developer container.
As an example, the specific flexure amount δ under the conditions under which the inventor measured was as follows:                Aluminum blank tube radius 8 mm thickness 0.6 mm→δ=0.080 mm        Aluminum blank tube radius 8 mm thickness 0.8 mm→δ=0.055 mm        Stainless blank tube radius 8 mm thickness 0.5 mm→δ=0.020 mm        
Here, the closest distance (hereinafter referred to as the “SD distance”) between the photosensitive drum 100 and the developing sleeve 9 will be considered.
As is well known to those skilled in the art, the SD distance is an important design parameter in the two-component developing method, and generally, it is often the case that the SD distance is set to a range of the order of 0.200 mm to 1.000 mm.
The more uniform is this SD distance, the better is secured the uniformity of image density. According to the result of the inventor's research and experiment, when an amount of fluctuation exceeds 10% relative to the center design value of the SD distance, the uniformity of image density becomes unallowable. Particularly, in a case where due to the unevenness of mass production, the SD distance is biased to the maximum value side of the tolerance, or a case where the layer thickness of the two-component developer on the developing sleeve 9 is biased to the minimum value of the tolerance, such non-uniformity of the SD distance is liable to be actualized as the non-uniformity of image density.
The degree to which the flexure of the developing sleeve 9 affects the SD distance depends also on the relation between the direction of flexure of the developing sleeve 9 and the direction of the central position of the photosensitive drum 100 as it is viewed from the developing sleeve 9.
For example, when in FIG. 6, the direction of flexure of the developing sleeve 9 is a leftwardly upward direction of 45 degrees and the photosensitive drum 100 is in a just rightward direction as viewed from the developing sleeve 9, the SD distance is changed by an amount corresponding to 0.7 δ of the SD distance direction component of the flexure amount δ of the developing sleeve 9.
That is, although depending on the construction of the developing apparatus 1 and the disposition relation thereof with the photosensitive drum 100, there is a case where due to the smaller radius of the developing sleeve 9, the SD direction component of the flexure amount δ thereof exceeds 10%, and it has been found that this is one of factors which have spoiled the uniformity of image density.